How do we choose the activation function for each hidden node?
To know the form of your non-linear function,  firstly you should define the type of problem you are dealing with such as an image classification task. Secondly, pick the activation functions based on your task such as sigmoid, Tanh, ReLu, LeadyRelu, Softmax etc. Overall, your ANN performance mainly depends on the number of hidden layers (hidden units), selection of activation functions,  weight decay, momentum and dropout etc.